The research proposed herein is to study the expression of murine sarcoma virus (MSV) genomes in transformed cells with the specific aims of identifying the segment of the genome encoding the transforming function and the gene product responsible for malignant transformation. Murine sarcoma virus genomes are recombinant molecules of murine leukemia virus (MLV) sequences covalently attached to cellular sequences. The map order of viral sequences and cellular sequences will be determined by nucleic acid hybridization studies using MSV- and MLV-specific cDNA as probes, and possibly electron microscopy. MSV-specific cellular RNA molecules will be extracted from transformed cells fractionated according to size and detected by nucleic acid hybridization with radiolabeled cDNase probe. Polypeptides encoded by the purified MSV-specific cellular and viral RNA molecules will be identified by in vitro translation in a messenger-dependent reticulocyte lysate. Attempts will be made to identify similar polypeptides in virus-transformed cells using 35S methionine to label the polypeptides in both cases and, where feasible, immunoprecipitation. The polypeptides will be analyzed on SDS polyacrylamide gels and autoradiography. Nontransforming mutants of MSV will be generated to aid in the identification of the polypeptide responsible for transformation, and the elucidation of its function. The Kirsten and Moloney isolates of MSV will be the focus of this research, which will later be extended to the other isolates of MSV.